This invention relates to a modified epoxy resin. More particulary, it relates to the modified epoxy resin containing a soft segment for use as a binder resin of cathodic electrodeposition paints.
Modified epoxy resins to be incorporated into cathodic electrodeposition paints are conventionally produced by reacting a starting epoxy resin with a cationic active hydrogen compound such as primary or secondary amines to open at least a portion of the epoxy rings with the active hydrogen compound in order to introduce a hydrophilic and electrically chargeable group into the resin. These resins may be thermally cured either by the reaction with an external crosslinker such as melamine resins or blocked polyisocyanates, or by the self-crosslinking reaction of a pendant blocked isocyanato moiety bound to a secondary alcoholic hydroxyl group present in the starting epoxy resin.
Since epoxy resins containing aromatic rings .such as biphenol epoxy resins generally produce a cured film having decreased flexibility, it is known that the flexibility may be improved by reacting the starting epoxy resin with an alkylphenol such as t-butylphenol or nonylphenol, or a long-chain fatty acid so that these components are introduced into the resin molecule by the ring opening reaction of a portion of epoxy rings with these components. Modified resins thus produced, however, have a defect that the curability thereof is decreased and the curing requires relatively high temperatures.
Another known approach for improving the flexibility is the use of an epoxy resin having a soft segment in the resin molecule. For example, bisphenol epoxy resins may be chain extended by the reaction with a bifunctional polyol containing the soft segment such as bifunctional polyether polyols. This approach has a defect that other performance of the cured film and corrosion resistance in particular are not satisfactory.